The use of carbon dioxide for enhanced production of oil and gas from reservoirs is well known. Liquefied gas based fracturing is unique as compared to conventional fluids such as water and have certain advantages in water sensitive and low pressure formations, including the promotion of fluid flowback (i.e., retrieval of water/fluid used in fracture treatment) which minimizes formation damage caused by water. Michael J. Economides, T. M. (2007). Modern Fracturing: Enhancing Natural Gas Production. (S. Weiss, Ed.) Houston, Tex., USA: Energy Tribune Publishing Inc. LCO2 used in fracturing treatments is typically added to a high pressure stream of water and proppant (typically solids, such as sand, polymer pellets, tracers, gravel, etc. of various sizes and density) at the well-head. Combining water with proppant and adding a separate pressurized LCO2 stream is the most conventional method of forming a CO2-energized fracture fluid. This is due, in large part, because it is simpler to mix proppant with water at atmospheric pressure then it is to add proppant to liquid carbon dioxide at a pressure above the triple point of carbon dioxide, (i.e., greater than 75.1 psia).
Equipment is available and can be used for small fracture treatments (e.g. to place up to approximately 20 tons of proppant) to mix proppant directly with a liquid carbon dioxide-based fracturing fluid. This equipment includes a pressurized vessel and manifold system that blends the proppant into a liquid CO2 stream prior to the high-pressure pumps. Proppant is loaded into the CO2 blender. The blender is sealed and then filled with CO2. During the fracturing process, proppant is mixed into the fracturing fluid by either hydraulically driven augers or gravity fed through a control valve. Michael J. Economides, T. M. (2007). Modern Fracturing Enhancing Natural Gas Production. (S. Weiss, Ed.) Houston, Tex., USA: Energy Tribune Publishing Inc. Once the batch of LCO2 and proppant is exhausted, the fracture treatment must either be completed or suspended to refill the blender with additional proppant.
Earlier efforts, as described in U.S. Pat. No. 4,374,545, provide for a batch process creating a proppant and LCO2 fracturing slurry. Each unit is capable of metering up to 20 tons of a single type of proppant and addresses the control of proppant supply through the use of a metering auger. LCO2 additions made to the bottom of the tank allow for a flowable and vapor-free proppant slurry leaving the system as well as maintaining pressure in the vessel.
Another system is described in U.S. Pat. No. 8,408,289 and U.S. Pat. No. 8,689,876 which depict an upright standing vessel where proppant is metered into LPG (liquefied petroleum gas) as a base fracturing fluid. Proppant loadings are varied into the LPG fracturing fluid stream through the use of gravity (through a control valve) or via one or more augers disposed within and along the bottom of the proppant supply source or arranged outside of the proppant supply source. Inert gas (in the form of nitrogen) is pumped into the vessel during operation to maintain vessel pressure to ensure the LPG mix remains in the liquid phase to prevent back flow into the vessel.
A non-mechanical pump, such as an eductor, can be used to mix a proppant into a fracturing fluid stream. Non-mechanical pumps have the benefit of no moving parts, are generally low cost and simple pieces of equipment, and are already commonly used in related material introduction. For instance, International Publication No. WO 2012087388 describes an eductor system for introducing and blending polymer additives into a fracturing fluid stream.
General use of a liquid eductor for solids handling and blending relies heavily on the relationship of motive flow (i.e., the incoming flow of fluid to the eductor (without proppant addition)) to the rate of solids entrainment for the control of solids concentration. As liquids pass through the converging nozzle of the eductor, potential energy is converted into kinetic energy resulting in a high velocity jet flow. This change in energy results in a localized decrease in static pressure that creates suction within the body of the eductor. This suction allows material to be drawn into the eductor and entrained by the fluid (LCO2, etc.). The eductor serves a dual purpose: mixing within the nozzle as well as drawing material into the fluid to ensure intimate mixing. With more conventional methods, such as using sand or similar material proppants to provide water-based slurries, the viscous properties of the water aids in drawing solid materials into the body of the eductor where suction occurs. Difficulty arises when it is necessary to establish a particulate suspension in a relatively low viscosity fluid (as compared to water), such as liquid carbon dioxide (LCO2). The present invention addresses the need to add proppant to such fluids on a more fully controlled basis by delivering a homogeneous fracturing fluid to high pressure pumpers prior to wellhead injection.
A system and method described in U.S. Pat. No. 7,735,551 is used to blend nitrogen gas with proppant to fracture an underground oil and gas formation or coal seam. The proppant and gas mixing occurs at a pressure sufficient to fracture the formation. In one embodiment, an eductor is employed to introduce proppant into the vapor stream and is in communication with the well bore. Proppant material is either gravity fed from a proppant reservoir into the eductor with the use of a control valve or regulated in with the use of an auger. The system described provides for the use of either valve position or auger speed to regulate proppant into the vapor stream to achieve specified proppant loadings. Pressure in the head space of the proppant reservoir is maintained at a constant value during the entirety of the stimulation.
To overcome the disadvantages of the related art, it is an object of the present invention to provide a control mechanism for operating a system for the delivery of proppant into a liquefied gas, such as LCO2, for the purpose of fracturing a subterranean formation. Although the liquefied gas discussed herein is in relation to LCO2, by way of example, it can be combination of immiscible and non-immiscible fluids such a CO2 and methanol, CO2 and biodiesel, or CO2 and water. Specifically, the control mechanism developed utilizes an eductor along with a proppant control valve and the pad pressure (as defined below) in the proppant reservoir to control proppant loading at specified concentrations in a substantially homogeneous fashion.
It is another object of the present invention to provide a system designed to mix proppant and fracturing fluid at pressures significantly below that of the surface treatment pressure (e.g. at or below 400 PSI).
It is yet another object of the present invention to provide a system where the eductor can be used with a liquid, and wherein said system does not utilize an auger for purposes of metering proppant into fracturing fluid.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.